There are a number of applications in which the measurement of the angular reflectance properties or the bidirectional reflectance distribution function (BRDF) of a material is important. These applications include materials characterization, tactile sensors for robotics, and fluid/aerosol particle size evaluation. One material characterization application involves the measurement of paint gloss. Conventional techniques for paint gloss measurement utilize optical sensors mounted on arms, such that the angle viewed by the sensor with respect to the surface can be changed. Such designs suffer from the fact they are not compact, and involve moving parts. Another material characterization application is the determination of the finish or the smoothness of a surface. For a machined part, the surface smoothness may be particularly important if the part is to be used in a press-fit joint. Typically, surface testing of this type is done by carrying out a visual comparison of the part to be tested with a series of standard samples.
In the robotics field, surface roughness measurements can provide significant benefits in tactile sensing. For example, it is known that a person estimates the force needed to hold an object by visually estimating the surface roughness of the object to be held. Thus an angular reflectance sensor that was sufficiently compact to mount within a robot finger could provide a surface roughness measurement to a robotics system, thereby permitting the robotics system to estimate the force required to hold an object. Such measurements could also of course be used to aid in identifying the object.
It is well known that the catastrophic failure of an engine, such as an aircraft engine, is usually preceded by a sharp rise in the particulate matter suspended in the engine oil in the 15-30 minutes before failure. It is also known that the size of particles suspended in oil determines the angular reflectance characteristics of the suspension. For example, large particles tend to produce retroreflection, whereas small particles tend to produce uniform angular scattering. Thus an effective sensor for measuring the angular reflectance of a suspension could find ready application in monitoring of engine oil for contaminants.